1. Field of the Invention
The present invention relates to a power tool, such as a disc grinder, an electric screwdriver, or a drill for boring, which is equipped with an electric motor therein as a power source.
2. Description of the Related Art
Such a power tool is generally equipped with either a gear train for changing the number of output revolutions of a motor or a gear train for changing the output direction. A CVT (Continuously Variable Transmission) that continuously varies the gear train and reduction ratio is commonly used as a transmission mechanism for power tools. Technology concerning CVT traction drives are disclosed, for example, in JP No. 6-190740 A, JP No. 2002-59370 A, and JP No. 3-73411 B2.
In a continuously variable transmission traction drive, a plurality of conical planetary rollers are supported by a holder. A centrally located sun roller is pressed onto the planetary rollers. A shift ring located around the holder is pressed onto the planetary rollers. Through rolling contact, planetary rollers transmit rotational power to an output shaft. The number of output revolutions is continuously altered due to the changing of the position of the shift ring relative to the planetary rollers. The pressing position of the shift ring pressed to the conical surfaces of the planetary rollers is varied between a small diameter and a large diameter.
A screw-tightening tool equipped with a continuously variable transmission therein is disclosed in JP 6-190740 A. In the screw-tightening tool, it is possible to continuously vary the speed and torque output. This is accomplished by moving a shift ring. In creating low speed/high torque output, thread-fastening can be easily performed.
A continuously variable transmission mechanism traction drive has various problems. In this type of continuously variable transmission mechanism, a transmission case is filled with a lubricant for transmitting power. Typical lubricants include traction oil or traction grease. Appropriate lubricants for use in CVTs have a relatively high viscosity in order to produce a high transmission coefficient (traction coefficient). During operation, the lubricant filled in the transmission case is scraped and supplied to the pressing portion of the planetary rollers. This occurs during the revolution of the planetary rollers and the rotation of the holder. This operation results in power transmission.
When the planetary rollers rotate, they in turn, scrape the lubricant. Accumulated lubricant unintentionally becomes stirring resistance in the power transmission system. As the load from this stirring resistance increases, the torque in the power transmission system decreases.
Accordingly, a power tool is desired that can suppress reduction of the output torque and/or prevent increase of load current in a motor. One method for accomplishing this is by reducing the stirring resistance of a lubricant, such as traction oil.